Porous membranes are used in a wide variety of engineering applications. Such membranes are often used in gas or vapor separation, reverse osmosis, electrochemical applications, hyperfiltration, ultra filtration and microfiltration. Such membranes can even be used for the manipulation of chemical reactions including selective ion separation.
During the last few years, several studies have evaluated the health risks associated with exposure to engine exhaust emissions. As a result of these studies, increasing government and health organizations have decided to tighten the standards which apply to engine-run vehicles, their fuels and their particulate and gaseous emissions.
On Nov. 15, 1990, the President of the United States signed into law the Clean Air Act Amendments of 1990. Beginning in 1994, the new law sets a performance criteria, particularly requiring buses operating more than 70 percent of the time in large urban areas (using any fuel) to cut particulates by 50 percent compared to conventional heavy duty vehicles. Also, beginning in 1994, the Environmental Protection Agency began requiring a yearly testing to determine whether buses subject to the standard are meeting the standard in use over their full useful life. Similar provisions exist in other countries and a global effort is underway to find exhaust falters and cleaning devices.
Because of the financial and logistical concerns with alternative fuels, transit authorities and bus engine manufacturers are seriously considering aftertreatment systems such as trap-oxidizer technology to meet 1993/94 EPA laws and regulations. Bus engines, for example, run on a stop-and-go cycle which forces the engines to operate with a dirty and sooty exhaust. Second, these vehicles operate in dense population areas and hence, bus exhaust and pollution is considered a greater health hazard than over-the-road trucks. Third, environmentalists would like to be as clean as possible even if it means going beyond EPA regulations. All of these factors make trap oxidizer technology very attractive, provided that its long-term durability can be proven and made available at reasonable costs.
The 1993/94 EPA law and regulations are only the first step in a series of ever-tightening regulations to follow. For the diesel engines industry, the next step in regulation occurs in 1998, when the laws require tighter NO.sub.x control. Even though NO.sub.x reduction for 1994 levels will be achieved by improved engine design, it is generally accepted that to meet the 1998 levels of 4 g/Bhp-h NO.sub.x, diesel engines will have to use aftertreatment systems. As the NO.sub.x level is reduced, however, the particulate level increases. Hence, in trying to meet the 1998 low NO.sub.x levels, engine manufacturers are faced with increased particulates, which require further use of aftertreatment devices such as converters and traps.
Aftertreatment devices of the present invention made of the micropyretically processed ceramic or ceramic composite porous membranes, as well as the micropyretically processed heating elements (which are essentially the ceramic or ceramic composite porous membranes), provide improved converters and trap systems which offer the flexibility, efficiency, and cost-effectiveness needed to meet the challenges presented by near-term (1994) and medium-term (1998) diesel emissions regulations. "Micropyretics" or "micropyretically synthesized," as used herein refers to self propagating high temperature synthesis as discussed in the review article by Subrahmanyam et at., in The Journal of Macromolecular Science at Vol. 27, p.p. 6249-6273 (1992).
The ceramic or ceramic composite porous membrane filters used in aftertreatment trap systems are the core of the system and great efforts are being made to fine-tune the existing systems to improve their effectiveness and durability. One of the problems of the current technology is the need for optimization of the filter structure, as in a modulated design. Another problem of the current technology is that currently available filter materials are not optimally thermally shock resistant nor are they highly thermally cyclable, as in reticulated micropyretically manufactured materials.
Presently, most of the available aftertreatment trap systems are based on the cellular cordierite ceramic monolith trap. These traps have not been efficient at collecting soot, and a large fraction of the particulate soluble organic fraction (SOF), and have several other limitations and leave much room for improvement.
For example, the straight channeled ("honey-comb") structure of the filters does not optimize stream line distortion nor surface area for particulate collection. Further, the dense wall cordierite ceramic used in today's filters is expensive.
The above described need for improved aftertreatment systems led to the invention of an alternative trap technology which was provided by the ceramic fiber coil traps developed by Mann and Hummel and Daimler Benz in West Germany. These traps were composed of a number of individual filtering elements, each of which consisted of a number of thicknesses of silica fiber yarn wound on a punched metal support. A number of those filtering elements were suspended inside a large metal can to make up a trap. However, failures during German field demonstrations appear to have reduced or eliminated work on that system, according to "Diesel Particulate Control Around the World," Michael P. Walsh et at. part of "Global Developments in Diesel Particulate Control" P-240, publ. SAE, Inc. (Febraury 1991).
Numerous other trapping media have also been tested or proposed, including ceramic foams, corrugated mullite fiber felts, and catalytic coated stainless steel wire mesh.
In addition to the problems of the presently available aftertreatment systems addressed above, the high concentration of soot per unit of volume in the ceramic monolith of the cellular trap makes these traps sensitive to "regeneration" conditions. Trap loading, temperature, and gas flow rates must be maintained within a fairly narrow window. Otherwise, the trap fails to "regenerate" fully, or cracks or melts to overheating because the high temperature gradients in the filter monolith damage the cordierite structure.
The most challenging problem of trap oxidizer system development has been with the process of "regenerating" the filter by burning off the accumulated particulate matter. Initiating and controlling the regeneration process to ensure reliable regeneration without damage to the trap is the central engineering problem of trap oxidizer development today. The reason is that over time, the filter becomes loaded with the soot it has trapped and must be cleaned or "regenerated". The process of regeneration burns or "oxidizes" the soot collected within the filter. The cleaned filter can be used many times provided it can be successfully regenerated many thousands of times over its lifetime without failure. Many different regeneration concepts are being tested. They range from primitive off-board regeneration of the filter in an external oven to sophisticated on-board automatic electrical or burner regeneration systems using electronic controls and include catalytic injection system. These approaches to regeneration can generally be divided into two groups: passive systems and active systems. Passive systems must attain the conditions required for regeneration during normal operation of the vehicle. Active systems, on the other hand, monitor the build up of particulate matter in the trap and trigger specific actions leading to regeneration when needed.
Passive regeneration systems face special problems on heavy duty vehicles. Exhaust temperatures from heavy duty diesel engines are normally low, and recent developments such as charge air cooling and increased turbo charger efficiency are reducing them still further. Under some conditions, it would be possible for a truck driver to drive for many hours without exceeding the exhaust temperature (around 400.degree.-450.degree. C.) required to trigger regeneration.
Active systems, on the other hand, are generally expensive, often requiring complex logic and electronics to initiate regeneration.
Engine and catalysts manufacturers have experimented with many catalytic converters and with a wide variety of regenerative catalytic traps. Precious metal catalytic traps are effective in oxidizing gaseous hydrocarbons and CO as well as the particulate SOF but are relatively ineffective in preventing soot oxidation, a particular problem for diesel engines. Moreover, these metals also promote the oxidation of SO.sub.2 to particulate sulfates such as sulfuric acid (H.sub.2 O.sub.4). Base metal catalytic traps, in contrast, are effective in promoting soot oxidation, but have little effect on hydrocarbons, CO, NO or SO.sub.2. Another disadvantage of precious metal catalysts is that they are very expensive.
Unlike a catalytic trap, however, a flowthrough catalytic converter does not collect most of the solid particulate matter, which simply passes through in the exhaust. The particulate control efficiency of the catalytic converter is, of course, much less than that of a trap. One of the major disadvantages of the catalytic converter is the same as with the precious metal catalytic particulate trap: sulfate emissions. The main object of the catalysts used is to raise the exhaust temperature to a point that could convert the gaseous compounds to safer gaseous emissions. The catalysts undergo chemical reactions which raise the temperature of the exhaust gases allowing them to be converted to the safer gases. One of the major reasons which catalytic material and treatments are used to assist in trap regeneration, is that none of the heating systems attempted, such as diesel fuel burners, electrical heaters and other heaters have been successful. However, if there were a regeneration system in which a converter or trap could be used without a catalyst for regeneration, the above-listed objects would be achieved.
With respect to processes for the manufacture of porous ceramic articles, U.S. Pat. No. 3,090,094, issued May 21, 1963 to K. Schwartzwalder et al, discloses a method of making an open-cell porous ceramic article which comprises immersing an open-cell spongy material, preferably polyurethane, in a slurry containing a ceramic coating material to coat cell-defining walls of the spongy material, removing excess slurry from the spongy material, and firing the coated spongy material at a temperature and for a time sufficient to remove the spongy material and form a hardened, vitrified structure. The ceramic coating material may include particulate zirconia, zircon, petalite, mullite, talc, silica and alumina, having particle sizes ranging from -80 mesh to -600 mesh. A binder such as clay, sodium silicate, and calcium aluminate and phosphoric acid, is preferably present in the slurry. Firing is conducted at 500.degree. to 3000.degree. F. (260.degree. to 1650.degree. C.), preferably at 2100.degree. to 2950.degree. F. (1150.degree. to 1620.degree. C.).
U.S. Pat. No. 3,097,930, issued Jul. 16, 1963 to I. J. Holland, discloses a method of making a porous shape of sintered refractory material which comprises impregnating a foamed plastic sponge shape with a suspension of refractory particles, drying the impregnated shape, and firing the dried shape in an inert atmosphere to volatilize the sponge material and to sinter the refractory particles. The impregnation and drying steps may be repeated. The foamed plastic sponge may be polystyrene, polyethylene, polyvinyl chloride, latex, or polyurethane, the latter being preferred. Refractory materials include clays, minerals, oxides, borides, carbides, silicides, nitrides and mixtures thereof. Specific examples used alumina, beryllia and china clay with particle sizes ranging from less than 1 to greater than 10 microns. Firing was conducted at 1700.degree. C. for alumina and 1350.degree. C. for china clay.
U.S. Pat. No. 4,697,632, issued Oct. 6, 1987 to N. G. Lirones, discloses a ceramic foam filter, insulating refractory lining, and a melting crucible, and a process for production thereof, which comprises providing an open-cell foam pattern, impregnating the pattern with a ceramic slurry, burning out the foam pattern at a temperature between 1400.degree. and 2200.degree. F. (760.degree. and 1205.degree. C.) to form a ceramic substrate, impregnating the ceramic substrate with additional ceramic slurry, and firing the impregnated ceramic substrate at a temperature of 2200.degree. to 3400.degree. F. (1205.degree. to 1870.degree. C.). The foam pattern material may be a flexible polyurethane, polyethylene, polypropylene or graphite. A suitable ceramic slurry contains from 1% to 20% silica (dry weight), and from 99% to 80% alumina (dry weight), with a viscosity between 5 and 20 seconds and a film weight between 1.0 and 8.0 grams per standard six inch square plate. Preferably the slurry includes a suspending agent, a wetting agent and a defoaming agent. Zirconia may also be used as ceramic material.
U.S. Pat. No. 3,111,396, issued Nov. 19, 1963 to B. B. Ball, discloses a method of making a porous metallic article which comprises impregnating a porous organic structure with a suspension of powdered metal, metal alloy or metal compound, and binder, slowly drying the impregnated structure, heating at about 300.degree.-500.degree. F. (150.degree.-260.degree. C.) to char the organic structure, and then heating at about 1900.degree. to about 3000.degree. F. (1040.degree. to 1650.degree. C.) to sinter the powder into a porous material.
Other United States patents relating to porous ceramic filters and methods for making them include: U.S. Pat. No. 3,893,917--Jul. 8, 1975--M. J. Pryor et al; U.S. Pat. No. 3,947,363--Mar. 30, 1976--M. I. Pryor et al; U.S. Pat No. 3,962,081--Jun. 8, 1976--J. C. Yarwood et al; U.S. Pat. No. 4,024,056--May 17, 1977--J. C. Yarwood et al; U.S. Pat. No. 4,081,371--Mar. 28, 1978--J. C. Yarwood et al; U.S. Pat. No. 4,257,810--Mar. 24, 1981--T. Narumiya; U.S. Pat. No. 4,258,099--Mar. 24, 1981--T. Narumiya; and U.S. Pat. No. 4,391,918--Jul. 5, 1983--J. W. Brockmeyer.
None of the above patents disclose or suggest the desirability of using conductive ceramic or ceramic composite porous membrane filters, which can also be used as heating elements. Additionally, there is no suggestion in any of the above patents to impregnate a substrate with a ceramic or ceramic composite slurry in the manner undertaken by the present invention. The problems associated with the prior art methods are similar to the problems associated with the method described in U.S. Pat. No. 5,279,737, which problems are described in greater detail below.
U.S. Pat. No. 5,279,737 ("the '737 patent") discloses a process for producing a porous ceramic, ceramic composite or metal-ceramic structure by micropyretic synthesis wherein a form polymer shape is impregnated with a slurry of ceramic precursors and ignited to initiate micropyretic synthesis, thereby attaining a ceramic, ceramic composite or metal-ceramic composite article having interconnected porosity. The '737 patent is incorporated by reference into the present application, in its entirety.
In the past, it was extremely difficult to incorporate a heating device into a filter because thermal cycling problems from incompatible thermal expansions of the heating element and filter. This made it difficult to have a filter with an integral heating element. The material used for the heating elements are typically molybdenum disilicide based. This material is able to heat to 550.degree. C. in only a few seconds, which combats the well-known problem of cold-start emissions in motor vehicles. Other superior properties include high emissivity of approximately 0.9, as compared to other heating elements which have emissivities of 0.4 to 0.75. The fact that the heating element is integral with the filter provides the advantage of less complexity, less moving parts and less cost. Most other systems depend upon many complex systems including logic and electronics to heat the filter or the exhaust gases, which is very costly and problematic. All materials made by the micropyretic technique experience a large temperature gradient of more than 1000 centimeters per millimeter during manufacture. This includes both filters and heating elements made by micropyretic technique. Due to the extreme conditions that the materials must endure during synthesis, the materials made by the micropyretic technique result in porous ceramics which are extremely thermally shock resistant, highly thermally cyclable and forgiving when contacted with the heating element. Because they possess these qualities, they are extremely well-suited for exhaust aftertreatment systems.
To date, such rapid heating elements were not available. Non micropyretic heating elements even though made principally of molybdenum disilicide or silicon carbide are extremely expensive. Furthermore, they cannot heat as rapidly because they are not manufactured by the micropyretic technique. Another reason why heating elements have not been used in situ with the types of filters most commonly used today, is that the extruded cellular configuration of the presently available falters is ill suited for integral heating elements. The extruded channels made of the ceramic act as an insulator with respect to the other channels. Therefore, one would need many heating elements, one per cellular channel to have an in situ heating configuration, a highly impractical and extremely expensive consideration.
The in situ heating elements would also enhance the catalytic converters already in use, today. Catalytic converters are heated during operation, and the EPA specifies a minimum time in which the catalysis bed must reach operating temperature. The standard solution has been to add an "pup" converter--a second, small converter upstream of the main unit. It acts like an igniter, and heats the exhaust stream rapidly, but little else about them is satisfactory. Even a small converter adds significant costs. It is often difficult to fit even a sizable converter into limited space, and it creates shielding problems by placing another source of intense heat close to engine components. All of these problems are overcome by the integral in situ heating element of the present invention.
Several different methods have been attempted to heat catalytic converters, including miniature radio transmitters that activate a non contact heating device heater, and additional catalysis. The present invention deals with the necessity of heating catalysts without the expense and complexity of the prior art.
There is also a greater need for technological improvements in catalytic converters and other engine emission reduction devices because there is a finite limit to the amount of platinum the most commonly used catalyst. Moreover, platinum is extremely expensive.
The idea of having an in situ heating element within a filter has many applications outside of exhaust systems, as well. One of these applications would be in a simple heating device. One of the major advantages is that the heating element is actually inserted into the filter rather than being supplied from outside the filter.
U.S. Pat. No. 5,094,075, issued Mar. 10, 1992 to Heinrich Berendes, discloses a particulate filter that can be regenerated by means of a burner working in the main engine exhaust stream. Regeneration is achieved by means of a burner to which fuel and oxygen-containing gas is supplied in a variable proportion. By this means, the burner produces the output required to achieve the regeneration temperature in the diesel engine. This patent requires an outside burner, instead of an in situ integral heating element, in order to regenerate the filter.
U.S. Pat. No. 5,015,381, issued May 14, 1991 to M. Edmund Eliion, et at, discloses a fluid filter element, filter, and process for its fabrication, wherein the filter element includes a flat base and a thin layer deposited thereupon having a channels though which a fluid may flow. In operation, the element is pressed against a flat surface, preferably against the backside of another element and a stack of filter elements, wherein the channels become closed conduits. The channels have a minimum requirement of thickness of the layer so that larger particles may not pass therethrough. Fabrication of the thin layer with the channels therein is preferably accomplished by masking a pattern corresponding to the channels and then vapor depositing the remainder of the thin layer, as masking and deposition permits actuated control of the heights of the deposited layer and then the minimum dimension of the channels. This patent does not disclose the modular design nor does it account for regeneration of soot and particulates.
U.S. Pat. No. 5,001,899, issued Mar. 26, 1991 to Enrique Santiago, et al, discloses a method and apparatus for cleaning of a soot filter in the exhaust line of a diesel engine with a combustion chamber placed in front of the soot filter where fuel nozzle and adapted electrical ignition method is built and thereby enabling the afterburning of the exhaust without secondary air. The exhaust in the combustion-chamber is mixed with the fuel which is injected through the fuel nozzle, and ignited by an ignition device with the existing portion of the unburned oxygen. The half exhaust effects the burndown of the accumulated soot in the soot filter. The apparatus disclosed herein is complex and requires many parts and does not teach a simple integral heating element to burn the soot in the filter.
"Regeneration Performance Of A Catalyst Versus Non-Catalyst Ceramic Membrane Diesel Particulate Trap", Rich Helfrich, et al, Global Developments and Diesel Particulate Control P-240 Society of Automotive Engineers, Inc., 121-132 (February 1991), describes a ceramic foam trap system using a parallel flow stacked element design. The individual elements are bonded together to form subassembly of 12 to 14 elements. The ceramic foam filter elements are non-reticulated material with a microporous membrane on a down stream (outer) side of the filter element. The trap itself has a center inlet through which the exhaust flows in the individual elements by way of the annular inlet ports (formed by adjacent elements). The elements in this invention are all the same size and each have the same function. The filtration of the gas in such a system is in a `parallel` fashion and such a filter system is clearly non-modulated as described and claimed herein.
U.S. Pat. No. 4,400,352, issued Aug. 23, 1983 to Ovea Rehnburg, et at, discloses a method and device for optimizing purification of diesel exhaust gases, the purification being carried out by a catalysis. This invention does not disclose modulated design nor does it disclose incorporating a heating element into the filter for regeneration.
"Gassing Truckers", The Economists Newspaper Limited, Business Finance and Science: Science and Technology: Pg. 97 describes a particle trap which works like filters in the exhaust pipe using two traps and switching between them, so one trap filters while the other one burns the collected particles. That article admits, that despite years of research, those traps were still unreliable, strongly indicating the need for reliable particle traps. Although the design described has two separate filter units, it does not teach the modular design of the present invention.
U.S. Pat. No. 5,334,570 discloses a porous catalyst support which may be used in a catalytic converter for treating automotive exhaust gases. The desirability of increasing "open frontal area" available for filtration is recognized. However, no mention or suggestion is made of increasing roughness to achieve this objective.
For the foregoing reasons, there is a need for an aftertreatment system of high effectiveness, low complexity and low cost, as well as a regenerating system incorporating a heating element (which is essentially a ceramic or ceramic composite porous membrane), integral with an exhaust ceramic or ceramic composite porous membrane filter, wherein both are highly thermally cyclable.